The present invention relates to a piezoelectric element, a fabrication method for the piezoelectric element, and an inkjet head, an inkjet recording apparatus and an angular velocity sensor including the piezoelectric element.
A piezoelectric material converts mechanical energy into electric energy or electric energy into mechanical energy. A typical example of the piezoelectric material is lead zirconate titanate (Pb(Zr,Ti)O3) (hereinafter referred to as “PZT”), that is, an oxide with a perovskite type crystal structure. In using the PZT with a tetragonal crystal structure in particular, the maximum piezoelectric displacement can be obtained along the <001> axis direction (i.e., the c-axis direction). However, most of the piezoelectric materials are polycrystals of aggregates of grains, and the crystal axes of the respective grains extend in various directions. Accordingly, the directions of the spontaneous polarizations Ps are various.
In accordance with recent downsizing of electronic equipment, there is a strong demand also for the downsizing of a piezoelectric element. In order to meet the demand, a piezoelectric element in the form of a thin film, which has a remarkably smaller volume than a conventionally frequently used sintered body, has become more occasionally used. Therefore, various developments and studies have been earnestly made for thinning the piezoelectric element.
For example, since the spontaneous polarization Ps of the PZT extends along the <001> axis direction, in order to realize high piezoelectric properties (piezoelectric displacement properties) even when the piezoelectric element is thinned, it is necessary to make the <001> axis of the crystal included in a PZT thin film extend vertical to one face corresponding to one end thereof along the thickness direction of a substrate. For this purpose, in conventional technique, on a monocrystal substrate of magnesium oxide (MgO) with a rock-salt crystal structure having the (100) plane on a top face thereof, a PZT thin film with good crystallinity and with the <001> axis oriented vertically to one face corresponding to one end thereof along the thickness direction of the substrate is directly formed at a temperature of 600 through 700° C. by sputtering using PZT as a target (for example, see Journal of Applied Physics, U.S.A., the American Institute of Physics, Feb. 15, 1989, Vol. 65, No. 4, pp. 1666–1670). This method is characterized by using the substrate of MgO monocrystal, and owing to this substrate, a piezoelectric thin film with high piezoelectric properties and preferred orientation along the crystal direction can be realized.
The MgO monocrystal is, however, a very expensive material, and therefore, this method is not preferred from the viewpoint of cost when it is applied to mass production of industrial products of piezoelectric elements using the piezoelectric thin film.
Therefore, a method for forming a crystal orientation film of a piezoelectric material on an inexpensive substrate is, for example, a sol-gel method, that is, a typical example of a post annealing method. Now, procedures for forming a crystal orientation film by the sol-gel method performed by using, for example, a SrTiO3 substrate will be described. First, on a RuO2 under electrode formed by the sputtering on the substrate, a sol solution including Zr and Ti in a concentration ratio of 75:25 is applied by spin coating, and the resultant is annealed and dried, so as to form a precursor film. Next, on this precursor film, several layers of a precursor film are formed by using a sol solution including Zr and Ti in a concentration ratio of 52:48, and thereafter, the resultant is burned at a high temperature of 900° C. In this manner, a PZT-based piezoelectric oxide thin film with the (001) crystal orientation is synthesized without causing a crack (see, for example, Japanese Laid-Open Patent Publication No. 2000-208828 (pp. 3–4)).
When the PZT thin film having the crystal orientation along the (001) plane corresponding to the crystal direction with a large piezoelectric constant is formed in the aforementioned manner, a piezoelectric thin film with high piezoelectric properties can be formed.
On the other hand, as a method for forming a crystal orientation film on an inexpensive substrate, the present inventors have developed a method for synthesizing the crystal orientation film without performing post annealing but employing, for example, sputtering. Now, procedures for forming a crystal orientation film by this method will be described. First, an electrode thin film made of a noble metal alloy of platinum (Pt) or iridium (Ir) including titanium (Ti) is deposited on a substrate as an underlying electrode by the sputtering. Next, a thin film of an oxide not including Zr and having the (001) crystal orientation, such as lead lanthanum titanate (PLT), that is, an oxide with the perovskite type crystal structure, is formed on the electrode thin film as an initial layer by the sputtering. Thereafter, a PZT film is deposited on the initial layer by the sputtering. Thus, a PZT thin film with the (001) crystal orientation can be obtained.
Furthermore, the present inventors have found that when an electrode thin film of a noble metal alloy including cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe) or copper (Cu) is used as the underlying electrode and a PZT film is directly formed on the electrode thin film, a PZT thin film with the (001) crystal orientation can be obtained.
Also when the PZT thin film with the crystal orientation along the (001) plane corresponding to the crystal direction with a large piezoelectric constant is formed in the aforementioned manner, a piezoelectric thin film with high piezoelectric properties can be formed.
Since the piezoelectric thin film formed in the aforementioned manner exhibits a large piezoelectric constant, large piezoelectric displacement can be caused even when the applied voltage is low, and therefore, such a piezoelectric thin film is expected to be used as an actuator in a variety of fields. Also, when a high voltage is applied to the piezoelectric thin film, further larger piezoelectric displacement can be caused.
When a high voltage is applied to an actuator in which a PZT film is formed by the sputtering not requiring the post annealing in the aforementioned manner, however, film peeling is disadvantageously caused between a film used as the underlying electrode and the perovskite type oxide film. Therefore, such an actuator has insufficient durability as a piezoelectric actuator for causing large displacement.